This invention relates to a fixed data reading system for microcomputers.
Among programs of microcomputers such as 4-bit ones, a series of fixed data are often prepared for related processings. For example, those data are applied to the table look-up for conversion of seven-segment codes for numerical displays or to an electronic musical instrument wherein fixed waveforms are memorized as digital data and data successively output are converted into analog signals through a digital to analog conversion. In a microcomputer, programs or fixed data such as described above have to be stored in a memory. Generally, a microcomputer includes a random access memory (hereinafter referred to as RAM) for storing alterable data and a read only memory (hereinafter referred to as ROM) for storing programs. The fixed data are usually stored in successive addresses of ROM. To have access to and read the programs and the fixed data stored in the ROM, it is necessary to discriminate the addresses for the programs and those for the fixed data. Usually, a program counter (PC) for specifying and address for the program and a data counter (DC) for specifying an address for the fixed data are provided. The addresses for the fixed data have to be specified by the contents of the data counter which consists of registers. In a 4-bit or any other bit microcomputers, the RAM, ROM, PC, DC, etc. are constructed in a single chip. Here, the provision of DC, which serves a similar role to that of PC, increases the hardware cost and the chip area.
Among the prior art, there is National Technical Report Vol. 25, No. 4, August 1979, T. Sakao et al "4-bit One-Chip Microcomputer MN1500 Series". This one-chip microcomputer has a "table look-up function", and the addresses for ROM data to the read out are specified by using the program counter and two other registers. The microcomputer also includes a read table instruction (RDTBL) as a data transfer instruction. This instruction has a function of transferring the ROM data specified by the contents of a high portion of the program counter and an extended accumulator, to the extended accumulator.